Polyglutamine neurodegenerative diseases represent a family of inherited disorders resulting from expansion of a CAG repeat coding for a glutamine tract in nine unrelated genes. The focus of our research is on one of these diseases, spinocerebellar ataxia type 3 (SCA3). Recent studies in the field show that understanding the normal function of polyglutamine proteins is very important for understanding pathogenesis in these diseases. Ataxin-3 (AfS), the protein mutated in SCA3, is a deubiquitylating enzyme (DUB). ATS appears to function in pathways regulating misfolded proteins. The working hypothesis for the proposed studies is that ATS monitors flow through the endoplasmic reticulum associated degradation (ERAD) pathway and through multiple protein interactions link ERAD to the aggresome-autophagy pathway;both its protective and pathogenic properties are hypothesized to be through these protein interactions. Specific Aims will: 1) characterize regulation of protein degradation in the ERAD pathway by wild-type and pathogenic ATS using in vitro reconstitution paradigms and cells and tissues from ATS knockout (KO) mice, 2) characterize ATS function in cellular stress from excess misfolded proteins and heat shock by examining protein complexes of ATS with key regulators of autophagy and heat shock and validating functions and pathology using cells and tissues from ATS KO mice and SCA transgenic mice, and 3) investigate potential protective and toxic properties of ATS DUB activity and ubiquitin interacting motifs (UIMs). In addition to providing insight into the relationship between protein function and pathogenesis in SCA3, these studies should generate important information on regulation of two critical degradation pathways, ERAD and autophagy, and how these pathways may be mechanistically linked and coordinately regulated through ATS dependent interactions. The proposed studies should generate considerable knowledge on the normal and pathogenic properties of ataxin-3, a protein that causes the fatal neurodegenerative disease, spinocerebellar ataxia type-3 (SCA3). A major focus of the study is to understand how ataxin-3 regulates key cellular pathways responsible for degrading misfolded proteins and how mutant ataxin-3 that is responsible for SCA3 may cause aberrant interactions and dysregulation of these pathways.